Using long-term water balances to parameterize the surface conductance in the Penman-Monteith equation to calculate evaporation at 0.05° spatial resolution

Evaporation from the land surface, averaged over successive 8-day intervals and at 0.05° (~5 km) spatial resolution, was calculated using the Penman-Monteith (PM) energy balance equation, gridded meteorology and a simple, 2-parameter, biophysical model for surface conductance Gs. This conductance is a function of evaporation from the soil surface, remotely-sensed leaf area index, absorbed photosynthetically-active radiation, atmospheric water vapour pressure deficit and maximum stomatal conductance (gsx). A unique value of gsx for each grid cell across the Australian continent was obtained by matching the mean annual-evaporation (E) calculated using the PM equation and a ‘Budyko-curve' hydro-meteorological model. First, the hydro-meteorological model was calibrated using long-term water balances from 285 gauged catchments. Second, gridded meteorological data were used with the calibrated hydro-meteorological model to estimate E for each grid cell. Third, the value of gsx for each cell was adjusted to equate E calculated using the PM equation with E from the hydro-meteorological model. This closes the annual water balance but allows the PM equation to provide a finer temporal resolution for evaporation than is possible with an annual water balance model. There was satisfactory agreement between 8-day average evaporation rates obtained using remotely sensed leaf area indices, the parameterized PM equation and observations of actual evaporation at four Australian eddy covariance flux sites for the period 2000 – 2008. This paper describes the model for Gs and the procedure used to calibrate the hydro-meteorological model and to evaluate gsx for each grid cell. Comparisons with flux station data are also presented.